The Salton Sea (Now You "Sea" It -- Now You Don't)

by Richard Busch

[Ed. Note: This article won first place in the adult
technical/advanced article competition in the American Federation
of Mineralogical Societies in 1995.]

The Salton Sea is a relative newcomer to southern
California. One hundred years ago, in the location currently
occupied by the Salton Sea, you would have found the bottom
of a dry lake. In fact, if you could climb into a time machine,
you would discover that the area now containing the Salton Sea,
known as the Salton Basin or Salton Trough, was alternately
flooded and dry. During at least one period of time in its
history, a significantly larger body of water occupied the Salton
Trough. Evidence of this inundation still exists, and can be
readily seen, in the surrounding hills.

During the early Pliocene age (about 8 million years ago),
the Salton Trough constituted the northern portion of the Gulf
of California. The former presence of marine (salt) water is
recorded in fossil-bearing sandy shales which, today, can be
found at the edges of the trough and in drill cores taken from
the central part of the basin at depths of over 10,000 feet.
Interestingly, the fossils found at both of these locations include
oyster shells; however, oysters live only in relatively shallow
water -- nothing more than a few hundred feet deep. So how
did oyster beds, which form in shallow water, wind up nearly
two miles beneath the present surface of the Salton Trough?

The answer can be found by examining the geology of the
Salton Trough. The Salton Trough is an example of what
geologists call a rift valley or
graben. A rift valley is a strip of land bounded
on opposite sides by roughly parallel faults. Through movement
of the faults, the strip of land sinks in a process termed
subsidence. In the case of the Salton Trough,
the land is bounded on the northeastern side by the San
Andreas, Sand Hills, and Calipatria Faults. The southwestern
side of the trough is bounded by San Jacinto, Coyote Creek,
and Superstition Hills Faults. Although the principal movement
of these faults is lateral (parallel to the surface of the earth),
there is also a vertical component to the movement. Thus, for
example, while the land to the southwest of the San Andreas
Fault moved about 200 miles northwest, it also dropped from
three to four miles in the process.

As the Salton Trough subsided, erosion from the
surrounding mountain ranges kept it filled with sediment. The
overall effect was to keep the top of the sediment within a few
hundred feet of sea level while the entire mass of sediments
sank to a depth of several miles. This explains why shallow
water oyster shells are found at depths of 10,000 feet in the
central part of the basin.

All of this raises a puzzling question: If the Salton Trough
was originally part of the Gulf of California, and if the basin is
currently several hundred feet below sea level, then why isn't
the Salton Trough and the Imperial Valley flooded with water
from the Gulf? In fact, most of the sediments overlying the
Pliocene rocks described earlier are non-marine (fresh water)
sediments. This means that, early on, the Salton Trough was cut
off from the Gulf of California. What happened?

What happened was the Colorado River. Flowing from
higher ground, the fresh-water Colorado River carried sediment
into the Gulf of California and the lower part of the Salton
Trough. Over time, the sediment accumulated into a delta (also
termed an alluvial plain) which eventually rose
above the level of the waters in the Gulf. Today, this alluvial
plain, which extends for a distance of 65 miles across the top of
the present-day Gulf of California at a minimum elevation of
40 feet above sea level, has kept the Gulf waters from flooding
the Salton Trough.

Geologic evidence shows that once the Colorado River's
alluvial barrier was in place, fresh water flowed from the river
into the Salton Trough filling it from time to time to varying
depths. As the river changed course, as most rivers do, the flow
would be diverted, in different proportions, to the Gulf and to
the Salton Trough. All the while, the trough subsided from
activity along the boundary faults. Erosional sediment poured
in from the surrounding mountains and from the Colorado
River to fill the resultant rift valley.

Many of the old shorelines are visible today in the
mountains surrounding the Salton Sea. There is one very
prominent shoreline at an elevation of 44 feet above sea level
which records the presence of Lake Cahuilla which once filled
the Salton Trough. Driving highway 86, along the southwestern
edge of the Salton Sea, one can clearly see the horizontal line
which separates the dark coating of calcareous tufa, which
formed below the water level, from the lighter rocks above.
Carbon-14 dating has shown that Lake Cahuilla existed up until
just a few hundred years ago.

In the latter half of the 19th century, the Salton Trough was
completely dry, or nearly so. Wallace Elliott, in his
History of San Bernardino and San Diego
Counties, dated 1883, mapped a "Dry Bed of Lake"
and described it thusly:

"Desert Formerly Fresh-Water Lake: A portion of the
Colorado Desert is many feet below the sea level, and there is
evidence that since the retreat of the gulf much of it has been
covered by a continuous sheet of fresh water. The evaporation
of moisture has, however, in modern times, exceeded the
precipitation, and most of the surface is now constantly dry;
though at the lagoons there still remains a miniature
representative of the wide-spread fresh-water lake which once
occupied the area surrounding them.

"[...] fresh-water shells -- anodonta, planorbis,
physa, and amnicola -- [show] plainly
the bed of a former lake. These shells are very abundant, the
Amnicoloe being sometimes drifted by the
wind till they cover and whiten the ground, and look like
miniature snow wreaths. They have generally lost their
epidermis, and the Anodontas are considerably
broken and decayed, but on the whole are so well preserved
that it seems impossible that many years have elapsed since
they were inhabited by living animals."

So what happened to cause the formation of the Salton Sea
in this "Dry Bed of Lake" within the last one hundred years?
Wallace Elliott provided the first hint:

"Dr. O. M. Wozencraft, of San Bernardino, has for years
entertained the idea of turning the waters of the Colorado
River, or a portion of them, into a system of canals for the
purpose of irrigating that portion of the Colorado Desert lying
below the level of the sea."

In 1891, Mother Nature started what Wozencraft proposed.
E. B. Preston, wrote in the Eleventh Report of the
[California] State Mineralogist (1893), that "In the
month of June, 1891, a steady flow of water entered the
depression from the southeast and continued to the northwest
uninterruptedly until an area 30 miles long and averaging 10
miles in width was covered to a depth of 6 feet."

To this body of water was given the name "Salton Lake."
The name was derived from the high saline content of the lake.
At first, an underground connection to the Gulf of California
was suspected to explain the salinity. Eventually it was
determined that the water entering Salton Lake was fresh, from
the Colorado River. The saltiness was due to the enormous rate
of evaporation which concentrated the tiny sodium chloride
content of the river. The salinity of Salton Lake was so great
that the New Liverpool Salt Company established a facility to
produce a fine quality of table salt from the water.

In 1901, the California Development Company built the first
canal system to divert water from the Colorado River to the
Imperial Valley for the purposes of irrigation. The canal system
functioned well for several years until, in 1905, unusually high
floods overwhelmed the canal system and destroyed the
regulating machinery. For a period of time the Salton Sea
received the full, uncontrolled flow of the Colorado River.

Over the next two years, the Southern Pacific Railroad
Company attempted to repair the breach. Underestimating the
power of the Colorado River, their control structures were
repeatedly washed away. Finally, in 1907, the river was sealed,
but only after 350,000 acres of land had been flooded. Today
some of that land has been reclaimed by the lowering of the
Salton Sea through evaporation. Presently, the Salton Sea, fed
by runoff from irrigation, has reached equilibrium with the rate
of evaporation and stands at 235 feet below sea level.

References

Elliott, Wallace W.;

History of San Bernardino and San Diego Counties,
California with Illustrations; Wallace Elliott Publisher;
San Francisco; 1883; pp. 173-174.

Descent to Mecca

by Richard Busch

For a spectacular, first-hand look at some of the non-marine
sediments which make up the "filling" of the Salton Trough,
take the automobile trip shown on the map below. Start by
heading east from Indio on Interstate-10 to the Cottonwood
Spring Road exit, then follow the road signs to the town of
Mecca. The 20-mile drive descends from 1600 feet above sea
level to 190 feet below sea level in Mecca.

Initially, the drive takes you past the Orocopia Mountain
foothills and Buried Mountain (when you see it, you'll know
why it was given that name). At the point where the road enters
the bottom of Box Canyon Wash, you'll get a clear view of the
fresh-water sediments which make up the 100-foot, near-vertical
walls of the canyon. Looking at the folded layers, tilted in every
direction, it is clear that significant geological activity has
occurred in the area. One of the causes of this activity is
located just ahead on the road. Box Canyon and the
surrounding hills terminate abruptly where the road crosses the
San Andreas Fault. This point, nearly at sea level, marks the
edge of the Salton Trough. Take highway 86 south, along the
western edge of the Salton Sea, to get a good look at the
ancient shoreline in the mountains to the west.

Regarding "Descent to Mecca"

Letter to the Editor by Don Layton

I wholeheartedly agree with Richard's saying that Box Canyon
Road, east of Mecca, California, is very worthwhile viewing
["Descent to Mecca;" Lithosphere; June
1995]. It's nice to get off the freeway and look at a rock up
close. I've driven both ways four or five times trying to make
sense of how the San Andreas Fault has contorted these
formations. For an equally spectacular, but different, view of
the erosive power of running water, take a look at the next
canyon north -- "Painted Canyon."

Just east of the orchards, vineyards, and Coachella Canal, turn
north onto a graded gravel road. The first three miles parallel
the Mecca Hills; then the road deteriorates as the next mile
and a half goes up the
"golly-gee-whiz-never-seen-anything-like-it" canyon. This
description is based upon a twenty-year-old memory of a
National Association of Geology Teachers (NAGT) field trip.
I plan to check it out in a standard highway vehicle this
September. The AAA maps of both Riverside and Imperial
Counties illustrate this location.

The preceding articles were originally published in the June
and September 1995
issues of Lithosphere, the official bulletin of the
Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch
(Editor).

Permission to reproduce and distribute this material, in
whole or in part, for non-commercial purposes, is hereby granted
provided the sense or meaning of the material is not changed and
the author's notice of copyright is retained.